In the art and practice of continuous lithographic printing, it is essential to continuously supply, in addition to the printing ink, an aqueous dampening solution to the printing plate or plates. The dampening solution forms a water layer in all of the non-image areas of the printing plate thereby disallowing transfer of ink from a separate ink input system of rollers to all but the intended image areas of the printing plate.
The dampening water in lithography is commonly supplied to the printing plate in the form of a dilute aqueous solution containing various proprietary combinations of buffering salts, gums, wetting agents, alcohols, fungicides and the like, which additives function to assist in the practical and efficient utilization of the various water supply and dampening system combinations that are available for the practice of lithographic printing. Despite their very low concentrations, typically less than about several percent, the salts and wetting agents have in practice been found essential if the printing press system is to produce printed copies having clean, tint-free background and sharp, clear images, without having to pay undue and impractical amounts of attention to inking and dampening system controls during operations of the press.
In the practice of lithographic printing, different proprietary formulations of dampening solutions are found to be of greatest utility depending largely upon the configuration of the dampening system. There is need for a dampening system that significantly reduces the apparent dependence of dampening efficiency upon the particular materials in the dampening solution.
A convenient way to describe all dampening systems, although this two-portion description is not often used in the trade, is to consider the two necessary operations portions:
a. The water input portion consisting usually of a chromium or cloth-covered pickup roller, or spiral-brush spray system, or spray nozzles and the like, as well as the tubes, tanks and controllers, which together convert an at rest bulk liquid dampening solution into a more or less continuous directionally-oriented, relatively thin film or fine mist of the solution, and PA1 b. The dampener portion consisting of a series of one or more rollers that receive and then convey the thin film or fine mist of water from the water input portion to a printing plate that is rotating at printing press speeds.
Dampening systems may also be classified according to whether the water being supplied at the printing plate cylinder of the press is supplied before or after the ink is supplied.
It is repeatedly claimed that water-first dampening is better for optimal printing quality that water-last dampening. In fact, most prior art dampening systems, when used in the water-last position cannot maintain the image differentiation at the printing plate that is essential to lithographic printing. The practical reason for these observations is that the film of water transferred to the plate by a water-last dampener is applied after the ink has been refreshed to the plate image areas. This water film may interfere with subsequent transfer of ink from the printing plate to the printing blanket and thence to the paper being printed, producing printed copies of inferior quality and in the extreme disallowing any ink transfer to the printing portions of the press. Another reason for selecting water first dampening is that water-last dampening systems tend to cause stagnation and water-logging of the ink that always resides on the rubber dampening form roller. This can result in ink-slinging or even set-off of ink onto the printing plate in non-image areas resulting in unwanted printed marks. The present invention addresses and eliminates these heretofore accepted restrictions of dampener location to water-first.